The present invention relates to a further improvement in the non-pressurized, dry and continuous method--using hollow-flight screw apparatuses--for regenerating salvaged rubber and to means for the implementation thereof, which means, under full adherence to the known advantages of said method, especially is ecological benefits, make possible a considerable improvement in the quality of the reclaimed rubber while, at the same time, guaranteeing an increased throughput for a given heat exchange surface, in other words, also for a given apparatus.
The invention employs the known basic principle referred to above of first depolymerizing the salvaged rubber, the same--as far as "reinforced" rubber is concerned--having been freed of any fabric plies and/or steel wire inserts and reduced to a suitable particle size of 1 to 5 mm, by heating in the presence of softeners and reclaiming agents such as NAFTOLEN and RENACIT and then subjecting it to rapid cooling. In the following the term "salvaged rubber" shall refer both to synthetic rubbers such as BUNA and the like as well as to used natural rubber or caoutchouc, which are also obtained in the form of discarded vehicle tires.
It is common knowledge that the regeneration cannot be initiated beneath a critical threshold temperature which, depending on the nature of the salvaged rubber, varies approximately between 150.degree. and 180.degree. C., and that if either the optimal reclaiming temperature or the optimal retention time at this or even higher temperatures is exceeded, "over-regeneration", with the accompanying severe reduction in quality, is the rsult. Furthermore, the reduced quality of the reclaimed rubber, which in all known cases is clearly inferior to that of new rubber, is attributable not least to the fact that always, in all known methods, not only the individual salvaged rubber particles but also the different zones of each particle are exposed to different temperatures for different lengths of time, with the result that over- and/or under-regeneration of a considerable proportion of the starting material has hitherto been inevitable.
In practice, for the regeneration of rubber, use is made today almost exclusively of the autoclave process. This process involves treating the salvaged rubber under pressure and heat with chemicals. This process, however, due not only to difficulty of operation and inefficiency but in particular to the considerable environmental pollution, is of no significance in developed countries. In addition to this, all the disadvantages of an extremely lengthy (5-12 h) discontinuous procedure are inherent in the method.
The only dry regeneration process, even if applied only to a very limited extent, is the extrusion process, during which the heat required for the regeneration is introduced into the salvaged rubber by way of convection and the simultaneous application of high pressure. Due to high installation and operating costs as well as to the poor quality of the reclaimed rubber, this process is virtually obsolete.